The present disclosure relates generally to operations performed and equipment utilized in conjunction with subterranean wells and, in particular, to subsurface safety valves having a reduced spread between opening and closing pressures.
Subsurface safety valves are well known in the oil and gas industry and act as a failsafe to prevent the uncontrolled release of reservoir fluids in the event of a worst-case scenario surface disaster. Typical subsurface safety valves are flapper-type valves that are opened and closed with the help of a flow tube moving telescopically within the production tubular. The flow tube is often controlled hydraulically from the surface and is forced into its open position using a piston and rod assembly that may be hydraulically charged via a control line linked directly to a hydraulic manifold or control panel at the well surface. When sufficient hydraulic pressure is conveyed to the subsurface safety valve via the control line, the piston and rod assembly forces the flow tube downwards, which compresses a spring and simultaneously pushes the flapper downwards to the open position. When the hydraulic pressure is removed from the control line, the spring pushes the flow tube back up, which allows the flapper to move into its closed position.
As the setting depths of such hydraulically-actuated subsurface safety valves continues to increase, the energy required to move the safety valve against the hydrostatic head acting on the hydraulic actuator also increases. For example, on conventional safety valves, suitable biasing means, such as a gas chamber or more usually a power spring, act on the hydraulic actuator to overcome the hydrostatic force. However, there are practical limits to maximizing biasing forces such as springs, and minimizing the hydraulic areas of a hydraulic piston and cylinder assembly. Generally, to move a small hydraulic piston and cylinder assembly against a high hydrostatic head requires a strong spring that results in a high “spread” in the operating pressure to move the safety valve from a first position to a second position. Increasing the spread requires a change in surface operating pressures. Moreover, the springs used in subsurface safety valves require very high pounds of force and length and therefore become quite expensive. Lastly, increasing the length of the valve to reduce the spread can also be costly as it requires a longer spring.